Organ cultures of embryonic rat tooth germs will be treated with the thymidine analog 5-bromodeoxyuridine (BrdU) in order to characterize the molecular mechanism(s) responsible for productive epithelial-mesenchymal interactions. It has been shown that BrdU arrests odontogenic differentiation in vitro without interfering with those macro-molecular activities necessary for cell viability. We therefore intend to resolve whether the inhibitory action of BrdU is manifested through epithelium, mesenchyme, or both. Furthermore, we will examine the distribution of BrdU in the DNA nucleotide sequences to determine whether any finite subset of genes is responsible for this highly specific termination of a genetically programmed process of differentiation.